1. Field of the Invention
The subject matter of this invention is related generally to motor controllers and is related more specifically to microprocessor-controlled motor controllers.
2. Description of the Prior Art
Motor controllers or motor starters as they are sometimes called are well known in the art. Generally, they comprises a switch or ganged switches which are operable to open or close to provide or interrupt current to the stator windings, for example, of an electric motor. The opening and closing process is controlled by appropriately arranged relay coils and relay contacts in what is well known in the prior art as a "relay ladder" arranged in a logical order to properly sequence the starting and/or stopping of the motor.
With the advent of the microprocessor it was found that part, if not all of the relay logic arrangement, could be replaced by a properly programmed microprocessor. Such arrangements are shown and described in a paper entitled "A Quantative Analysis of Grouped Single-Phase Induction Motors" published on page 125 of the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 2, March/April 1981 by J. R. Dunki-Jacobs and Robert H. Kerr; a paper entitled "Thermal Tracking--A Rational Approach to Motor Protection" by D. R. Boothman, E. C. Elgar, R. H. Rehder and R. J. Wooddall identified as IEEE Transactions Paper 2744029-5 recommended for presentation at the IEEE PES Winter Meeting, New York, N.Y., Jan. 27-Feb. 1, 1974; a paper entitled "Mircroprocessor-Based Universal Motor Protection System" appearing in the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 1, January/February 1981 by E. B. Turner and H. Michael Willey and in a descriptive bulletin (41-560(E)) entitled MOTOGARD.TM. for motor protection by the Westinghouse Canada Switchgear and Control Division issued January 1980. A reading of all of the above will show that certain factors are of importance when discussing motor starters or motor controllers.
One of the important features associated with a motor starter is the number of starts per given period of time which are allowable for a given motor. It has been found that it is necessary to limit the number of starts per hour or day, for example, for a motor starter because during the start-up of a motor considerably more heating takes place than under normal running conditions. In the past the number of starts have been limited electromechanically by utilizing an elaborate system of relays and mechanical timers. However the utilization of relays and timers in a system that is basically microprocessor-based means that more hardware is required. It would be advantageous if a motor controller system which was primarily microprocessor-based could utilize available input information including operator input criteria in conjunction with the available memory capability and microprocessor capability to prevent motor start-up in the event that a predetermined number of starts per unit time is exceeded. It would also be advantageous if means could be provided for indicating how much time the operator must wait once the number of starts per unit of time has been exceeded before a new start process will be allowed to take place.